Pressure and temperature swing adsorption processes and sorption cooling processes typically employ some adsorbent disposed in a metal vessel and on a metal screen or surface which provides support for the adsorbent and permits the adsorbent to be placed in contact with the fluid stream containing the adsorbable component over the range of conditions necessary for the adsorption and desorption. The metal structures and physical arrangement of these devices has placed certain process limitations which restrict the amount of adsorbent which actually comes in contact with the fluid stream, or is accompanied by heat transfer inefficiencies inherent in the disposition of the adsorbent.
Pressure and Temperature Swing Adsorption
Processes based on the selective adsorption of a fluid or a component of a fluid stream generally involve contacting the fluid stream with the selective adsorbent in an adsorption zone and the process is carried out in a series of steps. The adsorption zone is maintained at adsorption conditions favorable to selectively adsorbing a portion of the fluid stream or an adsorbable component of the fluid stream and producing an adsorption effluent which has a reduced concentration of the adsorbed component relative to the fluid stream. The adsorbable component is then desorbed from the adsorption zone by either reducing the pressure of the adsorption zone to a desorption pressure, or by increasing the temperature of the desorption zone to a desorption temperature. At the desorption conditions, the adsorbable component is purged from the adsorption zone. Following the desorption step, the adsorption zone is purged to remove the adsorbed component. The adsorption zone is then returned to the adsorption conditions by either cooling the adsorption zone or by repressurizing the adsorption zone. In addition to the above discrete steps, additional integration which link a first adsorption zone with another adsorption zone which is out of phase with the first adsorption zone are provided to conserve energy such as pressure equalization steps to reduce overall efficiency in pressure swing adsorption processes.
In pressure swing adsorption (PSA), a multi-component gas is typically fed to at least one of a plurality of adsorption zones at an elevated pressure effective to adsorb at least one component, while at least one other component passes through. At a defined time, the feedstream to the adsorber is terminated and the adsorption zone is depressurized by one or more cocurrent depressurization steps wherein pressure is reduced to a defined level which permits the separated, less strongly adsorbed component or components remaining in the adsorption zone to be drawn off without significant concentration of the more strongly adsorbed components. Then, the adsorption zone is depressurized by a countercurrent depressurization step wherein the pressure on the adsorption zone is further reduced by withdrawing desorbed gas countercurrently to the direction of the feedstream. Finally, the adsorption zone is purged and repressurized. The final stage of repressurization is typically with product gas and is often referred to as product repressurization. In multi-zone systems there are typically additional steps, and those noted above may be done in stages. U.S. Pat. No. 3,176,444 issued to Kiyonaga, U.S. Pat. No. 3,986,849 issued to Fuderer et al., and U.S. Pat. No. 3,430,418 and U.S. Pat. No. 3,703,068 both issued to Wagner, among others, describe multi-zone, adiabatic pressure swing adsorption systems employing both cocurrent and countercurrent depressurization, and the disclosures of these patents are incorporated by reference in heir entireties. The above-mentioned patents to Fuderer et al., and Wagner are herein incorporated by reference.
Various classes of adsorbents are known to be suitable for use in PSA systems, the selection of which is dependent upon the feedstream components and other factors generally known to those skilled in the art. In general, suitable adsorbents include molecular sieves, silica gel, activated carbon, and activated alumina. For some separations, specialized adsorbents can be advantageous.
In thermal swing adsorption (TSA) wherein the adsorbent undergoes a regeneration with steam or with a stream of heated fluid such as a slip stream of treated adsorption effluent from another adsorber. The steam or heated fluid is introduced to the adsorption zone in a desorption mode to desorb the adsorbed impurities and purge them from the adsorption zone. The desorbed impurities may be recovered, for example, by condensation of the hot regenerant stream as in the example of using steam as a regenerant, or disposed of by incineration when a heated fuel gas is employed as the regenerant. In a typical TSA installation, two or more adsorption zones are operated in an alternating manner to provide continuous treating wherein at least one adsorption zone is operating in an adsorption mode while another is operating in a desorption mode.
Generally, PSA and TSA processes are carried out with the selective adsorbent disposed in fixed beds and the fluid streams are passed through the fixed bed adsorption zones at varying conditions depending upon the particular cycle taking place. Often PSA cycles are limited by the hydraulics of the fixed bed which relate to the actual height of the bed, the adsorbent particle size, and the density of the adsorbent particle. Some TSA processes are carried out by disposing the adsorbent on a paper in an adsorbent wheel as exemplified in U.S. Pat. No. 4,402,717 to Izumo et al. In Izumo et al., an apparatus for removing moisture and odors from a gas stream comprises a cylindrical honeycomb structure made from corrugated paper, uniformly coated with an adsorbent and formed in the shape of a disk or wheel. The multiplicity of adsorbent-coated parallel flow passages formed by the corrugations in the paper serve as gas passage ways which are separated as a zone for the removal of water and odor causing components in the gas, and as a zone for the regeneration of the adsorbent. The zones for removal and regeneration are continuously shifted as the wheel is rotated circumferentially about its central axis. For example, monolithic or honeycomb structures rotate around either a vertical or a horizontal axis. Solvent-laden air flows through the wheel parallel to the axis of rotation. All but a small portion of the adsorbent is always removing water and odor causing components. The other (small) portion of the wheel is undergoing thermal regeneration--usually in the opposite flow direction. The wheel continuously rotates to provide a continuous treated stream and a constant concentrated stream. The coated wheel units suffer many disadvantages. They require a large physical space to accommodate the enclosure for the wheel having the multiple removal and regeneration zones, and the associated gas transfer equipment (fans and blowers). The adsorbent-coated paper has limited range of humidity and temperature within which it can maintain its structural integrity. This failure also limits the regeneration medium to dry, moderate temperature gases and air. The contact between the adsorbent and the gas stream and hence the adsorbent capacity for volatile organic compounds is limited to the very thin layers of adsorbent on the surface of the paper. U.S. Pat. No. 5,580,369, which is hereby incorporated by reference, discloses an adsorbent wheel which is composed of an organic synthetic paper support and an adsorbent dispersed in the paper support comprised of a Y-type zeolite blended with either silica gel, alumina, or X-type zeolite. U.S. Pat. No. 5,338,450 attempts to overcome problems associated with "adsorbent wheel" systems by employing a spiral-wound adsorber module which comprises a spiral adsorbent bed supported within a cylinder by an inlet screen and an outlet screen on opposite sides of the adsorbent bed and requires the fluid to pass through the adsorbent bed which is thermally regenerated.
Thermal swing adsorption systems are attractive when applied to concentrating somewhat more dilute mixtures of organic compounds and often achieve greater than 99% removal efficiency. Generally, thermal swing adsorption systems employ strong adsorbents--adsorbents with a strong affinity for adsorbing the trace component--for separating trace components which are present in the fluid stream. Typically the trace components in the feed stream are present in amounts less than about 3 mole percent. Thermal swing adsorption processes are also limited because the amount of purge gas required to both heat and sweep the adsorbent can be a considerable fraction relative to the amount of feed.
Pressure swing adsorption generally employs weak adsorbents and is used for separations wherein the amount of the component to be separated can range from about 5 mole percent to about 95 mole percent. PSA systems are preferred when high concentrations of valuable feedstock, products, or reusable solvents are to be recovered. A PSA cycle is one in which the desorption takes place at a pressure much lower than adsorption. In some applications, the desorption takes place under vacuum conditions--vacuum swing adsorption (VSA). To overcome the inherent low operating loadings on the weak adsorbent, pressure swing cycles generally have cycle times that are short--on the order of seconds to minutes--to maintain reasonably sized adsorbent beds.
Sorption Cooling
In the operation of sorption cooling systems, generally there are two or more solid beds containing a solid adsorbent. The solid adsorbent beds desorb refrigerant when heated and adsorb refrigerant vapor when cooled. In this manner the beds can be used to drive the refrigerant around a heat pump system to heat or cool another fluid such as a process stream or to provide space heating or cooling. In the heat pump system, commonly referred to as the heat pump loop, or a sorption refrigeration circuit, the refrigerant is desorbed from a first bed as it is heated to drive the refrigerant out of the first bed and the refrigerant vapor is conveyed to a condenser. In the condenser, the refrigerant vapor is cooled and condensed. The refrigerant condensate is then expanded to a lower pressure through an expansion valve and the low pressure condensate passes to an evaporator where the low pressure condensate is heat exchanged with the process stream or space to be conditioned to revaporize the condensate. When further heating no longer produces desorbed refrigerant from the first bed, the first bed is isolated and allowed to return to the adsorption conditions. When the adsorption conditions are established in the first bed, the refrigerant vapor from the evaporator is reintroduced to the first bed to complete the cycle. Generally two or more solid adsorbent beds are employed in a typical cycle wherein one bed is heated during the desorption stroke and the other bed is cooled during the adsorption stroke. The time for the completion of a full cycle of adsorption and desorption is known as the "cycle time." The heating and cooling steps are reversed when the beds reach the desired upper desorption temperature and lower adsorption temperature limits of the adsorption cycle. The upper and lower temperatures will vary depending upon the selection of the refrigerant fluid and the adsorbent. Some thermodynamic processes for cooling and heating by adsorption of a refrigerating fluid on a solid adsorbent use zeolite and other sorption materials such as activated carbon and silica gel. In these processes, the thermal energy from adsorbing zeolite in one place is used to heat desorbing zeolite located in another place. U.S. Pat. No. 4,138,850 relates to a system for solar heat utilization employing a solid zeolite adsorbent mixed with a binder, pressed, and sintered into divider panels and hermetically sealed in containers. The U.S. Pat. No. 4,637,218 to Tchernev relates to a heat pump system using zeolites as the solid adsorbent and water as the refrigerant wherein the zeolite is sliced into bricks or pressed into a desired configuration to establish an hermetically sealed space and thereby set up the propagation of a temperature front, or thermal wave through the adsorbent bed. The bricks used in U.S. Pat. No. 4,637,218 are preferably not more than 10 mm in thickness. U.S. Pat. No. 5,477,705 discloses an apparatus for refrigeration employing a compartmentalized reactor and alternate circulation of hot and cold fluids to create a thermal wave which passes through the compartments containing a solid adsorbent to desorb and adsorb a refrigerant. U.S. Pat. No. 4,548,046 relates to an apparatus for cooling or heating by adsorption of a refrigerating fluid on a solid adsorbent. The operations employ a plurality of tubes provided with parallel radial fins, the spaces between which are filled or covered with solid adsorbent such as Zeolite 13X located on the outside of the tubes. U.S. Pat. No. 5,518,977 to Dunne et. al., which is hereby incorporated by reference, relates to sorption cooling devices which employ adsorbent coated surfaces to obtain a high cooling coefficient of performance.
It is an objective of the present invention to provide a pressure swing adsorption system which is able to achieve fluid separations at lower differential pressures than conventional fixed bed PSA systems.
It is an objective of the present invention to provide a combined pressure and temperature swing adsorption system which is not limited by fixed bed hydraulics.
An adsorber module is sought which does not have the size limitations of the adsorbent wheel and has an increased adsorbent capacity for the removal of impurities from gas streams.
It is a still further object of this invention to provide an adsorbent module which is mechanically simpler to operate and is less costly to construct than fixed bed adsorbers and rotating desiccant wheels.
It is the object of the instant invention to provide an improved sorption cooling system for use in waste heat recovery, space heating, and air conditioning systems which is not limited by the regeneration efficiency of the adsorbent.